A retardation film is an optical film for realizing a wider viewing angle or the like of a liquid crystal display. The retardation value of the retardation film depends on wavelength. The wavelength dispersion of the retardation value is roughly classified into the following three kinds. The first is a retardation film which exhibits wavelength dispersion such that a retardation value is larger on the shorter wavelength side (hereinafter referred to as “normal dispersion”), the second is a retardation film which exhibits wavelength dispersion such that a retardation value scarcely changes ranging from the short wavelength side to the long wavelength side (hereinafter referred to as “flat dispersion”), and the third is a retardation film which exhibits wavelength dispersion such that a retardation value is smaller on the shorter wavelength side (hereinafter referred to as “reverse dispersion”).
Here, the normal dispersion and the reverse dispersion exhibit optical properties such that the wavelength dependence of the retardation value is large, and the flat dispersion exhibits optical property such that the wavelength dependence of the retardation value is small.
As the retardation film exhibiting the flat dispersion, a norbornene-based film (for example, trade name “ARTON FILM”, manufactured by JSR Corporation) obtained by film-forming and drawing a norbornene-based resin has been conventionally known. However, the norbornene-based film is so comparatively thick as approximately 60 to 80 μm that, accordingly, a liquid crystal display having this optical film becomes comparatively thick and heavy.
On the other hand, it is known that a retardation film containing polyimide may be formed by coating a solution, which contains the polyimide, on a base material, and the retardation film containing polyimide exhibits predetermined retardation value (Patent Document 1). In the case where the retardation film is formed by coating, the obtained retardation film can be formed comparatively thinly. However, the retardation film containing the polyimide ordinarily exhibits the normal dispersion (that is to say, the retardation film does not exhibit the flat dispersion).
The retardation value in the thickness direction of the retardation film is determined by the expression “Δnxz×thickness”. Even if a retardation film having a large Δnxz is comparatively thin, the retardation film exhibits predetermined retardation value. Therefore, a film having a large Δnxz is demanded.    [Patent Document 1] Japanese translation of PCT international application No. 2000-511296